Cooling means for power tubes



April 18, 1967 B. SHRADER COOLING MEANS FOR POWER TUBES Filed July 21, 1964 mun-111mm United States Patent 3,315,107 COOLING MEANS FOR POWER TUBES Merrald B. Shrader, Leola, Pa., assignor to Radio Corporation of America, a corporation of Delaware Filed July 21, 1964, Ser. No. 384,117 12 Claims. (Cl. 313-39) The present invention relates to electron tubes capable of relatively high power output, and particularly concerns means for cooling internal parts of such tubes.

One type of power tube in which the invention finds utility is a tetrode include an external anode, a cathode and two grids between the anode and cathode. The electrodes mentioned are tubular and have concentric active regions. The two grids include a control grid around the cathode and a screen grid around the control grid.

During operation of a power tube of this type, electrons emitted by the cathode pass through apertures in the con trol and screen grids on their way to the anode. The apertures are defined by wires or wire-like strands that are disposed in the path of the emitted electrons. Some of such electrons, therefore, strike the closer control grid, and although the wires or strands of the more remote screen grid are usually disposed in the shadow of the control grid wires or strands, an appreciable number of electrons reach the screen grid, particularly since the screen grid is usually operated at a potential more positive than the potential of either the cathode or control grid. One effect of electron impingement on the wires or strands of the grid is to raise the temperature of the grids.

As the power rating of a tube is increased, the amount of electron emission from the cathode is also increased, with a consequent increase in the temperature of the c0ntrol and screen grids. When this temperature reaches a certain value, the grids emit primary electrons. This is objectionable for several reasons. The primary emission from the grids adversely aifect a desired control of the cathode emission for which the grids are designed. If this emission progresses, it finally terminates in what is known as a runaway condition. In some instances, the temperature of the grids may become so high at or prior to the runaway condition so as to cause the wires or strands of one or both of the grids to melt.

In one type of tetrode power tube, as described in Patent 2,980,984, issued to M. B. Shrader et al., and assigned to the assignee of this application, control and screen grids are mounted at one group of adjacent ends only, on metal supports that extend outside of the tube envelope and are adapted to be connected to circuit elements. Such connection serves to dissipate an appreciable amount of the heat in the portions of the grids adjacent to the aforementioned ends thereof, to the circuit elements. However, the dissipation of heat from the adjacent mounted ends of the control and screen grids, has a negligible effect upon the temperature of portions of the grids spaced from the mounted ends thereof. When the rated operating power of the tube is exceeded, these uncooled portions of the grids are subject to temperature increases that may reach the runaway condition aforementioned.

It is thus apparent that it is desirable, and it is an important object of the invention, to cool portions of a control and screen grid in a tetrode that are otherwise free to reach objectionable temperature levels.

It is a further object of the invention to provide an electron tube of the tetrode type in which portions of the control and screen grids thereof within the envelope of the tube, are effectively cooled by common cooling means.

Another object is to provide a vacuum tight chamber within the envelope of the tube, wherein the major walls of the chamber are formed by portions of two electrodes to be cooled, and duct means communicating with the chamber for circulating a coolant through said chamber.

A further object is to provide elongated duct means sealed through the envelope of an electron tube and connected to an end portion of an electrode within the envelope, and wherein harmful stresses resulting from differential expansions of the duct means and the electrode in response to heat, are substantially eliminated.

An exemplary embodiment of the invention comprises an electron tube having two tubular grids supported in cantilever fashion at one group of adjacent ends by flanges extending through the tube envelope. The other, or inner group of adjacent ends terminate within the tube envelope in spaced relation with respect to the envelope wall. In accordance with this invention, the inner ends of the grids define flat walls extending transversely of the grids and axially spaced from each other. A ring made of insulating material is sealed to the facing surfaces of the flat walls to form a vacuum tight chamber. Two ducts are sealed in mutually spaced relation to one of the flat walls, to provide a circulating path for a coolant through the chamber. Since the chamber is constituted of a portion of each of the two grids, both portions of the grids adjacent to the chamber are effectively cooled by the coolant. The ducts may be made of a material matching the coeflicients of expansion of the grids, or that may b mounted on expansion compensating structures.

The cooling structure described has permitted a substantial increase in the power rating of tubes in which it is used, i.e., in one example, from 3 kw. (C.W.) to 10 kw. (C.W.).

In the drawing to which reference is now made, for an exemplary embodiment of the invention,

FIG. 1 shows a sectional elevation of an electron tube in which the invention is embodied and in which coolant ducts are sealed through the envelope header of the tube;

FIG. 2 is a fragmentary sectional view of a flexible seal between one end portion of a coolant duct and a wall portion of a tube envelope;

FIG. 3 is a fragmentary sectional view of a flexible seal between the other end portion of a coolant duct and a wall of a chamber to be cooled;

FIG. 4 is a transverse sectional View, taken along line 44 of FIG. 1;

FIG. 5 is a fragmentary view in sectional elevation and shows the coolant ducts sealed through the anode of the tube;

FIG. 6 is a fragmentary view in section showing a plate fixed to a wall of the chamber formed by electrode ends to be cooled, for reinforcing the wall and for facilitating the sealing of a coolant duct to said wall; and

FIG. 7 is a fragmentary sectional view of a modified form of flexible seal between a coolant duct and a wall through which it extends.

The tube shown in FIG. 1 comprises an envelope formed in part by an external anode 10. The anode 10 may be made of copper, for example, and has a top wall 12 provided with a closed exhaust tubulation 14. A radiator 16 surrounds the anode 10 for dissipating heat therefrom either by radiation or by a forced flow of a coolant therethrough. The lower end of the anode 10 is fixed as by brazing in a vacuum type manner to a flanged metal ring 18, made of copper or an alloy known as Kovar, for example. The flange of ring 18 is in turn fixed as by brazing to a flange of a metal ring 20, which also may be made of copper or Kovar. The ring 20 is fixed to a ceramic tube 22, made of aluminum oxide for example, in a ceramic-to-metal seal. The two rings 18, 20, provide a flexible joint between the anode 10 and the ceramic tube 22 to accommodate different magnitudes of radial expansion of these elements. An insulating envelope header 21 made of aluminum oxide for example, and a plurality of mutually insulated annular sealed regions between electrode support flanges 24, 26, 28,

effected by the ceramic tube 22, the envelope header 21, and insulating rings 30, 32 made of aluminum oxide for example, complete the envelope of the tube.

Within the tube envelope referred to, are three electrodes. These electrodes comprise a tubular mesh cathode 34 supported on a frusto-conical support 36. The cathode 34 may be made of a wire mesh formed in a manner described in Patent 3,104,841 to C. T. Johnson et al., and assigned to the assignee of this application. The wire mesh of the cathode may be made of nickel suitably coated with an electron emitting material such as a mixture of the oxides of barium, strontium and calcium. The frusto-conical support 36 and an upper reinforcing ring 38 for the cathode 34, may be made of a material such as copper. The wire mesh of the cathode 34 is suitably brazed to the support 36 and to the ring 38. The support 36 may be integral with flange 28. The cathode 34 is a directly heated type adapted to be heated by a power source, not shown, connected across flange 28 and a conducting rod 40, made of molybdenum for example, sealed through the envelope header 21 in a ceramic to metal seal. The conducting rod 40 is provided with a transverse arm 42 (FIG. 4) fixed as by brazing to the ring 38 engaging the upper end of the cathode. Additional arms may be provided for increased ruggedness. If desired, the cathode 34 may be of the indirectly heated type with a suitable heater disposed therein. However, where the coolant ducts extend through the cathode, as shown in FIG. 1, a directly heated type of cathode is preferred since this involves less crowding of the space within the cathode.

Surrounding the cathode 34 is a control grid 44 having a plurality of longitudinal wires or strands 46 formed by the erosion technique described in the aforementioned Patent 2,980,984. The top of grid 44, as viewed in FIG. 1, is formed by a plate 48. The grid 44 may be integral with a lower frusto-conical support 50 terminating in flange 26. The support 50 and flange 26 may be made of a metal such as copper.

Surrounding the control grid 44 is a screen grid 52 having longitudinal wires or strands 54 effectively positioned in the shadow of strands 46 of control grid 44, by the method described in Patent 2,980,984, previously referred to. The screen grid 52 may be integral with a frusto-coni-cal support 56 terminating in flange 24. The support 56 and flange 24 may be made of a material such as copper. The upper end of the screen grid 52, as viewed in'FIG. 1, is closed by a plate 58.

It will be seen from the foregoing that when the exterior portions of flanges 24, 26, are connected to conducting circuit elements, heat in the lower portions of the grids 44, 52, is adapted to be dissipated effectively by conduction to such element-s. However, no effective way of dissipating heat from the upper ends of the grids has been available prior to the present invention.

The end plates 48, 58 of the two grids serve as walls of a chamber or partly closed space 60 by interposing an insulating ring 62 between facing surfaces of the end plates. The ring 62 is positioned relatively close to the perimeter of the end plates 48, 58 so that the end plates form a relatively large portion of the walls of chamber 60 for a reason that will become apparent. The ring 62 is suitably sealed to the end plates 48, 58 in a vacuum tight manner.

Means are provided for circulating a fluid coolant, such as deionized water, through the chamber 60. This means may take the form of ducts 64, 66 sealed vacuum tight at one end through openings in the lower end plate 48, and at their other end regions, partly through envelope header 21. The ducts 64, 66 may be made of a metal such as copper sealed at their lower ends in seals 68, 70 within the material of the envelope header 21 to two insulating tubes 72, 74, made of aluminum oxide, for example, extending outside of the tube envelope. By making the portions 72, 74 of the ducts of insulating ma- 4 terial, there is reduced danger of accidentally affecting the potential on the control grid 44. The tubes 72, 74 are adapted to be connected to conduits of a coolant circulating system, not shown.

It will be seen in FIG. 1 that the circulating coolant favors end plate 58 more than it does end plate 48. This is because the normal flow of the coolant urges it against the end plate 58 more than against end plate 48. Furthermore, end plate 48 is of appreciably smaller surface area than end plate 58, because of the openings in end plate 48 through which the ducts 64, 66 are sealed. In view of this situation, it is preferred that the insulating ring 62 be made of a material that is both electrically insulative and heat conductive, such as beryllium oxide. In this way, both end plates 48, 58 are substantially uniformly cooled.

In the embodiment shown in FIG. 1, the ducts 64, 66 and the electrode elements shown are predominantly made of the same material which may be copper. Therefore, axial expansions due to heat are substantially uniform throughout the internal structure. This uniform expansion renders feasible the rigid seals between the coolant ducts and the members they engage. However, when the ducts 64, 66 are made of a material characterized by a different coefficient of expansion than that of the other elements of the internal structure of the tube, the ducts may be joined to the end plate 43 and/or to the envelope header 21, through expansion compensating means. When the expansion compensating means is included in the joint between the coolant ducts and header 21, it may take the form shown in FIG. 2. In this form, an annular sheet metal member 76 is provided with a flange 78 which is fixed to a duct 64, 66 in a vacuum tight seal as by brazing. Another flange St) is fixed as by brazing to the inner surface of the header 21. The ducts 64, 66 are joined in end-butt seals to insulating duct portions 72, 74 in the region that may lie within the cavity formed by expansion compensating member 76. The bend 84, in this member, permits flexures therein, thus allowing for diflerent axial temperature response of the coolant ducts 64, 66, and other portions of the internal structure of the tube.

A flexible joint between a coolant duct 64 and end plate 48 of the control grid 44, is shown in FIG. 3. In this arrangement, a flexible member 86, which may be similar to member 76 of FIG. 2, is fixed to the underface of end plate 48 and to the upper end portion of a duct 64 in vacuum tight seals.

In some applications, one flexible joint of the type described for each duct may be suflicient. However, in situations involving relatively large differences in expansion between the ducts 64, 66 and the other internal elements of the tube, flexible connections of the type described may be desirable at both ends of the ducts.

The provision of the aforementioned expansion compensating means renders it feasible to provide coolant ducts made of a material selected independently of its expansion characteristic. Thus, where the space within a cathode through which the coolant ducts extend is crowded, it'may be desirable to use a material that is electrically insulating, such as aluminum oxide, for the ducts to avoid electrical shorts that otherwise may be likely to occur therein. Indeed, where the ducts extend through a cathode, it is preferred to employ a duct material that is made of thermally insulating material to avoid drawing heat from the cathode. For this purpose, a material such as aluminum oxide is preferred because of its relatively small thermal conductivity. Beryllium oxide may be used, but because of its relatively high thermal conductivity, is less suitable. While capacitive effects are relatively small within a cathode of the type described, the use of an electrically insulating material instead of metal as the composition of the coolant ducts, contributes to a substantial elimination of any capacitive loading by the ducts.

It will be seen from the foregoing discussion and in the discussion to follow, that a number of seals or joints are involved in the assemblies shown in the drawing. Some of these seals or joints are effected between metal elements and others between metal and ceramic elements. One way in which the metal-to-metal seals or joints may be made is by interposing between the parts a relatively high temperature brazing material, such as is known commercially as Nioro, which is an alloy of nickel, iron and gold. This brazing material has a melting point of 950 C., which is appreciably above the highest operating temperature of the tube. In making ceramic-to-metal seals, conventional techniques may be used, such as metalizirig the ceramic surface to be sealed with molybdenum and nickel plating the molybdenum. The nickel plating forms a good bond with the brazing material. If desired, washers made of Kovar (not shown) may be interposed in the seal regions between the ceramic and metal parts, to provide graded seals.

In the example shown in FIG. 1, the fluid circulating ducts 64, 66, have portions 70, 72, extended through the envelope header 21 and sealed to openings in the lower plate, as shown in FIG. 1. It is also feasible to modify the position of the coolant ducts so as to cause the ducts to extend through the anode top wall 12, as shown in FIG. 5. In this modification, coolant ducts 86, 88 are sealed at one group of adjacent ends through or in axial register with openings in the plate 58, closing the upper end of the screen grid 52. Metal eyelet 90 may be used to ruggedize the seal regions between the ducts 86, 88 and the end plate 58. The other end portions of the coolant ducts 86, 88 pass through oversize openings 92 in the anode top Wall 12 and are sealed to expansion compensating structures 94 which may be similar to structure 76, shown in FIG. 2. Instead of metal eyelets 99, a metal plate 94, shown in FIG. 6, may be brazed to the upper surface of screen grid plate 48 for ruggedizing this plate and for contributing to convenience in sealing the coolant ducts 86, 88 to the plate. In this connection, the openings 96 through plate 58 may be equal substantially to the internal diameter of the ducts 86, 88, while the openings 98 in the ruggedizing plate94 may be slightly larger than the outer diameter of the ducts to accommodate a high temperature brazing material therein such as Nioro.

Since the chamber defined by the insulating rings 62 and the end plates 48, 58 of the two grids, is at substantially atmospheric pressure within an evacuated ambient, it is desirable that the end plates referred to be sufficiently rugged to successfully withstand the pressures involved. Thus, if it should be impractical initially to form the end plates 48, 58 to the thickness required for desired ruggedness, each of plates 48 and 58 may be reinforced by a ruggedizing plate 94. Where a ruggedizing plate is used in connection with the control grid plate 48, it is preferably fixed to the lower surface of plate 48 as viewed in FIG. 6, to avoid encroachment upon the space within the chamber 60.

In the embodiment of FIG. 5, the space between the screen grid plate 58 and the anode top plate 12, is characterized by relatively strong electrical fields. Any metal element disposed in these fields gives rise to objectionable capacitive loadi'ng. To reduce such capacitive loading, the coolant ducts 86, 88 are made of an insulating material such as aluminum oxide.

In FIG. 7, there is shown an expansion compensating structure that may be used to advantage when the expansion differential between elements engaged by the structure is severe. The structure, as shown, effects a seal be tween coolant duct 86 and the top wall 12 of the anode. The compensating structure comprises two axially spaced annular rings 100, 102 sealed to duct 86 and anode top wall 12 respectively. The rings 100, 102 have flanges sealed to a bellows 104. The bellows 104 may be made of relatively thin stainless steel stock.

While the facing surfaces of the grid end plates 48, defining the chamber 60, as shown in FIGS. 1, 5 and 6, depicted as being smooth, such surfaces may be modito effect increased thermal transfer therefrom to the are coolant used. To this end, the inner surfaces may be roughened and/or baffles or fins (not shown) may be fixed thereto.

The cooling structure described is relatively rugged so that a coolant may be circulated therethrough at appreciable pressure and velocity, thereby contributing to an effective thermal dissipation from the otherwise inaccessible portions of control and screen grids within the tube envelope.

What is claimed is:

1. An electron tube having:

(a) an evacuated envelope,

(b) a chamber within said envelope defining an ambient at substantially atmospheric pressure,

(c) two electrodes within said chamber,

(d) said chamber having walls constituting portions of said two electrodes, and

(e) duct means communicating with said chamber and sealed through a wall of said envelope and adapted to pass a coolant through said chamber.

2. An electron tube having:

(a) an envelope having an evacuated ambient,

(b) a chamber within said envelope defining an ambient at a different pressure than said evacuated ambient,

(0) two electrodes within said chamber,

(d) said chamber having walls in heat transfer relation with said two electrodes, and

(e) duct means communicating with said chamber and sealed through a wall of said envelope and adapted to pass a coolant through said chamber.

3. An electron tube having:

(a) an evacuated envelope,

(b) a chamber within said envelope defining an ambient at substantially atmospheric pressure,

(c) two electrodes within said chamber,

(d) said chamber having walls constituting portions of said two electrodes, and

(e) duct means communicating with said chamber and including a portion sealed through a wall of said envelope and adapted to pass a coolant through said chamber,

(f) said portion of said sulating material.

4. An electron tube having:

(a) an evacuated envelope including an envelope header,

(b) a chamber within said envelope defining an ambient at substantially atmospheric pressure,

(c) two electrodes within said chamber,

(d) said chamber having walls constituting portions of said two electrodes, and

(e) duct means communicating with said chamber and sealed through said envelope header .and adapted to pass a coolant through said chamber.

5. An electron tube having:

(a) an envelope having a portion defining an anode,

(b) a chamber within said envelope defining an ambient at substantially atmospheric pressure,

(c) two electrodes within said chamber,

(d) said chamber having walls constituting portions of said two electrodes, and

(e) duct means communicating with said chamber and sealed through said anode.

6. An electron tube having:

(a) an envelope,

(b) two coaxial tubular grids in said envelope, having self-supported adjacent end portions,

(c) coaxially and axially spaced grid plates closing said end portions of the grids and having facing surfaces,

(d) a ring of insulating material sealed to and between said facing surfaces and forming a chamber with said surfaces, and

(e) two coolant ducts sealed through one of said grid plates and communicating with the interior of said chamber, said coolant ducts being sealed through a duct means being made of inwall of said envelope and including portions extending externally of said envelope and adapted to be connected to a source of coolant fluid.

7. An electron tube having:

(a) an evacuated envelope,

(b) two electrodes in said envelope, said electrodes including regions remote from the inner walls of said envelope,

(c) said regions of said electrodes defining two walls of a flat chamber at substantially atmospheric pressure whereby said chamber has a relatively large lateral dimension, and

((1) means for circulating a coolant through said chamher,

(1) said means comprising two ducts connected to portions of said chamber spaced laterally thereof.

8. An electron tube having:

(a) an envelope,

(b) two coaxial tubular grid assemblies in said envelope, said grid assemblies having adjacent ends fixed to said envelope, the other ends of said grid assemblies being self-supporting,

(c) coaxially and axially spaced grid plates closing said other ends of the grid assemblies and having facing surfaces,

((1) a ring of insulating material sealed to and between said facing surfaces and forming a chamber with said surfaces,

(e) two coolant ducts having different coefiicients of expansion than said grid assemblies and extending parallel to said assemblies, and sealed through one of said grid plates and communicating with the interior of said chamber, said coolant ducts being sealed through a wall of said envelope and including portions extending externally of said envelope and adapted to be connected to a source of coolant fluid, and

(f) expansion compensating means engaging said ducts intermediate said envelope wall and said one of said end plates.

9. An electron tube having:

(a) an envelope having an evacuated ambient,

(b) two coaxial tubular grids in said ambient, said grids having one group of adjacent ends fixed to said envelope, the other ends of said grids being self-supporting,

'(c) coaxially and axially spaced grid plates closing said other ends of the grids and having coextensive facing surfaces,

(d) a ring of insulating material sealed to and between said facing surfaces and forming a chamber with said surfaces adapted to have an ambient of a different pressure than said evacuated ambient, and

(e) two coolant ducts sealed through only one of said grid plates and communicating with the interior of said chamber, said coolant ducts being sealed through a wall of said envelope and including portions extending externally of said envelope and adapted to be connected to a source of coolant fluid,

(f) said insulating material of said ring being thermally conductive for substantially equalizing the temperature of said grid plates.

10. An electron tube comprising:

(a) an envelope having an evacuated ambient,

(b) a cooling chamber within said envelope and spaced from the walls of said envelope and closed from the evacuated ambient of said envelope,

(c) two parallel elongated electrodes supporting and in heat transfer relation to, said chamber,

((1) duct means extending through a wall of said envelope in parallel relation to said electrodes and communicating with the interior of said chamber,

(c) said electrodes and said duct means having different coefficients of expansion, and

(f) expansion compensating means means and one of said envelope walls and said chamber.

11. An electron tube comprising:

(a) an evacuated envelope,

(b) a chamber within said atmospheric pressure,

(c) said chamber comprising coextensive regions,

(d) an insulating ring sealed to facing surfaces of said walls at said coextensive regions and closing the sides of said chamber and,

(e) coolant duct means sealed through only one of said walls for directing a coolant flow into said chamber, whereby said coolant flow impinges upon the other of said walls,

(f) said insulating ring being made of a thermally conductive material for substantially equalizing the cooling effect on both sides of said walls,

(g) each of said two walls of said chamber being supported by and in heat transfer relation with a diff ferent electrode in said envelope.

12. An electron tube having:

(a) an evacuated envelope,

(b) two electrodes in said envelopes, said electrodes including adjacent regions having lead-ins extending through a wall portion of said envelope and other adjacent regions within said envelope remote from said lead-ins,

(c) said other regions of said electrodes defining two opposite walls of a chamber at substantially atmospheric pressure, and r (d) means for circulating a coolant through said chamber.

envelope atsubstantially two spaced walls having References Cited by the Examiner UNITED STATES PATENTS 1,230,708 6/1917 Hewitt 313-32 FOREIGN PATENTS 120,738 1/1944 Australia.

DAVID J. GALVIN, Primary Examiner,

between said duct 7 

12. AN ELECTRON TUBE HAVING: (A) AN EVACUATED ENVELOPE, (B) TWO ELECTRODES IN SAID ENVELOPES, SAID ELECTRODES INCLUDING ADJACENT REGIONS HAVING LEAD-INS EXTENDING THROUGH A WALL PORTION OF SAID ENVELOPE AND OTHER ADJACENT REGIONS WITHIN SAID ENVELOPE REMOTE FROM SAID LEAD-INS, (C) SAID OTHER REGIONS OF SAID ELECTRODES DEFINING TWO OPPOSITE WALLS OF A CHAMBER AT SUBSTANTIALLY ATMOSPHERIC PRESSURE, AND (D) MEANS FOR CIRCULATING A COOLANT THROUGH SAID CHAMBER. 